untitled

118 CHAPTER 6

NO 3 –

nitrate

NO 2 –

nitrite

nitrate

reductase

glutamine

glutamine

synthase

NH 4 +

ammonium

glutamate

glutamate

dehydrogenase

nitrite

reductase

HOOC.CH 2 .CH(NH 2 ).COOH+

HOOC.CH 2 .CO.COOH

α-ketoglutaric acid

+

CH 3 .CO.COOH

CH 3 .CH(NH 2 ).COOH

alanine

glutamic acid +

+

pyruvic acid

Mineral nutrient requirements: nitrogen,

phosphorus, and iron

Fungi need many mineral nutrients in at least trace

amounts (Table 6.1) but nitrogen, phosphorus, and iron

merit special mention because of their significance for

fungal activities and interactions in nature.

Nitrogen

Of all the mineral nutrients, nitrogen is required in the
largest amounts and can often be the limiting factor
for fungal growth in natural habitats (Chapter 11).
Fungi do not fix atmospheric N 2 but they can use many
combined forms of nitrogen. We will discuss this in
relation to the enzyme-mediated sequence below,
which represents the normal pathway for assimilation
of nitrogen by fungi and many other organisms.

All fungican use amino acids as a nitrogen source.
Often they need to be supplied with only one amino
acid such as glutamic acid or glutamine, and from this
they can produce all the other essential amino acids
by transaminationreactions. The standard form of
these reactions is shown below, using as an example,
the production of alanine from glutamic acid, or vice
versa:

Most fungican use ammonia or ammonium (NH 4 ) as
a nitrogen source. After uptake, ammonia/ammonium
is combined with organic acids, usually to produce either
glutamic acid (from α-ketoglutaric acid) or aspartic
acid; then the other amino acids can be formed by
transamination reactions, as noted above. The relat-
ively few fungi that cannot use ammonium and thus
depend on organic nitrogen sources include some
water moulds (Saprolegnia and Achlya spp.), several
Basidiomycota, and the mycoparasitic Pythiumspp.
such as P. oligandrum(Chapter 11). However, ammonia/
ammonium is often not an ideal nitrogen source in
culture media, even for many of the fungi that can use

it. The reason is that NH 4 +is taken up in exchange

for H+, and this can rapidly lower the pH of a culture

medium to 4.0 or less, inhibiting the growth of many

fungi.

Many fungican use nitrate as their sole nitrogen

source, converting it to ammonium by the enzymes

nitrate reductaseand nitrite reductase. The fungi

that cannot use nitrate include Saccharomyces cerevisiae

and many Basidiomycota, whereas some other fungi

such as Neurospora crassaare induced to express the

genes for nitrate reductase and nitrite reductase only

when a preferred nitrogen source is unavailable.

By referring to the nitrogen assimilation pathway

(above), it is clear that any fungus that can use NO 3

(or another nitrogen source towards the left of this

pathway) must also be able to use the other forms of

nitrogen towards the right. But, the regulatory controls

on nitrogen uptake ensure that nitrogen sources are

not necessarily used in the ways we might expect. If a

fungus is supplied with a mixture of nitrogen sources,

then ammonium is taken up in preference to either

nitrate or amino acids. The reason is that ammonium,

or glutamine which is one of the first amino acids

formed from it, prevents the synthesis of membrane-

uptake proteins for other nitrogen sources, and also

prevents the synthesis of enzymes involved in nitrate

utilization.

Phosphorus

All organisms need significant amounts of phosphorus,

in the form of phosphates, for production of sugar phos-

phates, nucleic acids, ATP, membrane phospholipids,

etc. But phosphorus is often poorly available in nat-

ural environments, because even soluble phosphate

fertilizers are soon rendered insoluble when they com-

plex with organic matter or with calcium and magne-

sium ions in soil. Plant roots, in particular, have

difficulty in extracting phosphorus from soil, because

they deplete the small pool of soluble phosphate in their

immediate vicinity and then have to depend on the

slow solubilization and diffusion of phosphate from

further away. By contrast, fungi are highly adept at

obtaining phosphorus, and they achieve this in several

ways ( Jennings 1989):

• They respond to critically low levels of available
  phosphorus by increasing the activity of their
  phosphorus-uptake systems;


• they release phosphatase enzymes that can cleave
  phosphate from organic sources;


• they solubilize inorganic phosphates by releasing
  organic acids to lower the external pH;


• their hyphae, with a high surface area/volume ratio,
  extend continuously into fresh zones of soil.